Terminal devices for conventional upper limb prostheses usually afford only one or two gripping modes. For example typical body-powered hook type terminal devices are capable of finger tip and key type modes of grip but are not as well suited for holding larger cylindrical type objects or objects that have tapered or other odd shaped configurations. The usual terminal devices associated with myoelectric prostheses afford a basic three-chuck type of grip mode but for the most part can additionally accommodate only objects that are larger than the minimum size opening between the closed thumb and finger elements of this type of terminal device. Such limited variety of gripping actions or modes normally associated with conventional terminal devices thus restricts the range of object holding activities that can be conveniently carried out by a prosthesis wearer.
Many attempts have been made to increase the functional versatility of terminal devices but such attempts even where partially successful have to varying degrees rendered such devices too heavy or mechanically complex for practical use. This condition represents just another example of the supremely difficult task associated with designing a terminal device that possesses most of the desired characteristics and features, such as being lightweight, strong, aesthetically acceptable, functionally versatile, relatively low cost, durable, mechanically simple and easy to maintain. Further it is most desirable that such a device be easy to repair in the field with simple tools by personnel needing very little in the way of skilled training. As may be readily appreciated many of these structural and functional objectives become virtually mutually exclusive in attempts for their attainment and such accounts in large measure for the relatively slow progress over the years in significantly enhancing the mechanical efficiency of such terminal devices.